Real planet 'Tatooine' - with two suns
Published 3:31 pm, Thursday, September 15, 2011
Mountain View --
Astronomers hunting for distant planets far off in the Milky Way with the spacecraft Kepler have discovered a single planet flying in an oddball orbit around two sun-like stars at once - the first double-solar system ever seen, and there may be many more, they say.
The discovery offers a rare opportunity to study the evolution of such so-called binary systems and the planets that are born with them, says Laurance R. Doyle of the SETI Institute in Mountain View, a member of the planet detection team at NASA's nearby Ames Research Center.
Astronomers like Doyle are calling their newfound planet "Tatooine" instead of its stodgy designation Kepler-16b, because it recalls their own youth as "Star Wars" fans and the fictional planet that was Luke Skywalker's home - where "the sands of Tatooine bake beneath the glare of two bright suns," as it says in the video game "Star Wars: the Old Republic."
"Sooner or later," Doyle says, "science fiction becomes science fact, and if you were on the planet you'd be seeing two sunsets at a time."
But there are no baking sands on this distant planet, for it is obviously bitter cold, and reminiscent of our own planet Saturn, according to Doyle's research.
Astronomers around the world have been seeking so-called "exoplanets" beyond our solar system ever since the first one was detected in 1995. Already, more than 645 have been detected. But all are orbiting single stars, not two at once as this one is.
Gas, rock and ice
The newfound planet is about 65,000 miles in diameter - more than eight times the Earth's. Analysis shows it is probably half gas - hydrogen and helium - and half rock and ice. It orbits its two parent stars every 229 days at a distance of about 65 million miles and appears to be about 200 light-years away from us, according to Doyle.
The stars in what's called this "eclipsing binary system" are much smaller than our sun, and are about 42 million miles apart. As the planet passes in front of each star in turn, the planet dims the light of each, while the two stars in the binary system eclipse each other again and again.
"This demonstrates that planets can indeed form around double star systems," Doyle said in a conversation this week, "so I think the next thing is to look for habitable planets around eclipsing binaries and add them to the search list- there may be about 2,000 of them bright enough to be visible in Kepler's field."
Because the planet and both stars lie in the same plane - just as the planets do in our own solar system - Doyle suggests that all three could have evolved together from a single whirling disk of gas and dust and rock, perhaps about 3 billion years ago.
A report on the new discovery is published today in the journal Science, with Doyle the lead author and nearly 50 other scientists collaborating.
Kepler's quest for distant planets is led by William Borucki at the Ames center and the team uses a specialized telescope aboard the spacecraft, which is now scanning some 155,000 stars in the Milky Way constellations Cygnus and Lyra.
Kepler's telescope spots distant planets by measuring how a star's light dims almost imperceptibly whenever a planet passes in front of it, and that dimming can be measured precisely by Kepler's instruments to determine the size and mass of the sun. Or, as in this case, the two suns.
Both stars are much smaller than our sun, Doyle said - one about two-thirds the size of ours, the other only 20 percent as big. The larger one is bright orange, and the smaller is deep red.
Borucki's team has so far identified 1,235 possible exoplanets - with 54 of them circling their suns in "habitable zones" - at distances that could be just right for those planets to hold liquid water, and, just possibly, some form of life. An additional 500 are expected to be announced next month.
In this case, the two stars in this "binary system" are also dimming each other's light as each revolves around its partner every 41 days, over and over. So while the planet flies around the system, first its shadow dims the light of one star, then it dims the other, and finally at a certain point in its orbit, it dims both stars at once.
Since Kepler has demonstrated that planets can indeed form around double-star systems, Doyle says, the next thing is to look for systems like these in "habitable zones" where planets might hold life - perhaps like Luke Skywalker's old home on Tatooine.
"This is a wild system," said Geoffrey Marcy, the UC Berkeley astronomer and pioneer in the quest for these exoplanets who is one of the report's co-authors. "It's a head-spinning carousel of a planetary system."